Crystal structure of the co-crystal salt 2-amino-6-bromo­pyridinium 2,3,5,6-tetra­fluoro­benzoate

Bosch, E.

doi:10.1107/S2056989019001294

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Volume 75| Part 2| February 2019| Pages 284-287

https://doi.org/10.1107/S2056989019001294

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Crystal structure of the co-crystal salt 2-amino-6-bromopyridinium 2,3,5,6-tetrafluorobenzoate

Eric Bosch ^a ^*

^aDepartment of Chemistry, Missouri State University, Springfield, MO 65897, USA
^*Correspondence e-mail: ericbosch@missouristate.edu

Edited by S. Parkin, University of Kentucky, USA (Received 9 January 2019; accepted 23 January 2019; online 31 January 2019)

The asymmetric unit of the co-crystal salt 2-amino-6-bromopyridinium 2,3,5,6-tetrafluorobenzoate, C₅H₆BrN₂⁺·C₇HF₄O₂⁻, contains one pyridinium cation and one benzoate anion. In the crystal, the aminopyridinium cationic unit forms two hydrogen bonds to the benzoate oxygen atoms in an R₂²(8) motif. Two pyridinium benzoate units are hydrogen bonded through self-complementary hydrogen bonds between the second amine hydrogen and a carboxylate O with a second R₂²(8) motif to form a discrete hydrogen-bonded complex containing two 2-amino-6-bromopyridinium moieties and two 2,3,5,6-tetrafluorobenzoate moieties. The 2-amino-6-bromopyridinium moieties π-stack in a head-to-tail mode with a centroid–centroid separation of 3.7227 (12) Å and adjacent tetrafluorobenzoates also π-stack in a head-to-tail mode with a centroid–centroid separation of 3.6537 (13) Å.

Keywords: crystal structure; 2-amino-6-bromopyridine; 2,3,5,6-tetrafluorobenzoic acid; 2-amino-6-pyridinium 2,3,5,6-tetrafluorobenzoate; hydrogen bond.

CCDC reference: 1893117

1. Chemical context

The fields of crystal engineering and supramolecular chemistry rely on the identification and application of versatile synthons to guide the construction of molecular solids (Desiraju, 1995 , 2013 ). For example carboxylic acids are known to form a centrosymmetric dimer through self-complementary O—H⋯O hydrogen bonds (Fig. 1a) in addition to hydrogen-bonded catemer chains and rings. It has been shown that these hydrogen bonds can be diverted by O—H⋯N hydrogen bonding to pyridines, often supported by a non-conventional pyridine C—H⋯O hydrogen bond (Fig. 1b). The interaction of the more basic pyridines, for example 4-(N,N-dimethylamino)pyridine, with carboxylic acids most often yields charge-assisted hydrogen-bonded salts (Fig. 1c). Similarly, the combination of 2-aminopyridines and benzoic acids has been demonstrated to be a reliable supramolecular synthon resulting in the formation of charge-assisted hydrogen-bonded complexes shown in Fig. 1d (Bis & Zaworotko, 2005 ). The formation of hydrogen-bonded co-crystals or salts of amines and acids has potential in the pharmaceutical field where the physicochemical properties of active pharmaceuticals, including aqueous solubility and physical and chemical stability, may be modulated and tailored by co-crystal or salt formation (Schultheiss & Newman, 2009 ). For example a study involving the non-steroidal anti-inflammatory drug piroxicam reported the formation of 19 pyridine based co-crystals (Wales et al., 2012 ). The present study presents the first co-crystal/salt formed between a substituted pyridine and 2,3,5,6-tetrafluorobenzoic acid.

Figure 1
Potential hydrogen-bonding motifs for (a) carboxylic acid dimers, (b) neutral hydrogen-bonded pyridine carboxylic acid co-crystals, (c) charge-assisted pyridinium carboxylate hydrogen-bonded complexes, and (d) charge-assisted 2-aminopyridinium carboxylate hydrogen-bonded complexes.

2. Structural commentary

The asymmetric unit of the co-crystal salt 2-amino-6-bromopyridinium 2,3,5,6-tetrafluorobenzoate (I), contains one pyridinium cation and one benzoate anion that are held together by two charge-assisted hydrogen bonds (Table 1, first two entries) to form an R₂²(8) motif (Fig. 2). The bond distance C12—O2 is slightly shorter than C12—O1, with distances of 1.236 (2) and 1.267 (2) Å respectively. The atoms that form this R₂²(8) motif (Fig. 1) are almost coplanar, with the maximum deviation above and below the least-squares plane calculated through all of these atoms being 0.169 (7) and −0.147 (8) Å, respectively, for O2 and O1. The angle between the planes defined by the benzene and pyridine rings is 67.04 (7)° and the carboxylate anion is twisted out of the plane of the benzene ring, with C12 0.103 (3) Å above the plane of the benzene ring and O1 1.043 (3) Å above, and O2 0.713 (4) Å below the plane defined by the benzene ring.

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
N1—H1⋯O1	0.91 (2)	1.68 (2)	2.585 (2)	177 (2)
N2—H2A⋯O2	0.87 (2)	1.98 (2)	2.845 (2)	175 (2)
N2—H2B⋯O2ⁱ	0.87 (2)	2.02 (2)	2.854 (2)	162 (2)
C9—H9⋯Br1ⁱⁱ	0.95	2.94	3.867 (2)	166
Symmetry codes: (i) -x+1, -y+1, -z+1; (ii) -x, -y+1, -z+1.

Figure 2
The molecular structure of the co-crystal salt (I)

showing the atom-labeling scheme. Displacement ellipsoids drawn at the 50% probability level and hydrogen bonds (Table 1

) are shown as dotted lines.

3. Supramolecular features

In the co-crystal salt (I), adjacent amino pyridinium benzoate salt units are linked into dimeric salt complexes with self-complementary hydrogen bonds (Table 1, entry 3) from the second amine hydrogen atom and carboxylate oxygen atom O2 in a second R₄²(8) motif (Fig. 3). The two components are relatively well separated within the crystal structure into zones parallel to the c axis.

Figure 3
Part of the crystal structure of (I)

viewed along b, highlighting the hydrogen-bonded dimeric salt unit.

There are two interactions that involve the tetrafluorobenzoate (Fig. 4). Adjacent tetrafluorobenzoates π-stack in a head-to-tail mode with a Cg1⋯Cg1ⁱ distance of 3.6537 (13) Å [symmetry code: (i) −x, 1 − y, z; Cg1 is the centroid of the benzene ring C6–C11] and there is a close C—F⋯π interaction with a Cg1⋯F3ⁱⁱ distance of 3.1640 (17) Å [symmetry code: (ii) −x, y − $[{1\over 2}]$ , $[{1\over 2}]$ − z].

Figure 4
Partial view of the packing in the crystal structure of (I)

highlighting the head-to-tail π-stacking of the tetrafluorobenzoate molecules and the C—F⋯π interaction.

The 2-aminopyridinium groups form offset alternating head-to-tail π-stacks parallel to the b axis (Fig. 5) with a Cg2⋯Cg2ⁱⁱⁱ distance of 3.7227 (12) Å and a shortest perpendicular interplanar distance of 3.2547 (8) Å [symmetry code: (iii) 1 − x, y − $[{1\over 2}]$ , $[{3\over 2}]$ − z; Cg2 is the centroid of the pyridine ring].

Figure 5
Partial view of the packing in the crystal structure of (I)

highlighting the head-to-tail π-stacking of the 2-amino-6-bromopyridinium cations.

There is one short contact to the bromine with a C9⋯Br1^iv distance of 3.867 (2) Å [symmetry code: (iv) 1 − x, 1 − y, 2 − z].

4. Database survey

A search of the Cambridge Crystallographic Database (Version 5.39, update of August 2018; Groom et al., 2016 ) using Conquest (Bruno et al., 2002 ) for structures including the neutral carboxylic acid dimer synthon as shown in Fig. 1a revealed 6,016 hits, while a search for neutral pyridine carboxylic acid interactions where the distance between the acid proton and the pyridine N is equal to or less than the sum of the van der Waals radii revealed 2189 hits. In 966 of the 2189 structures the distance between the carbonyl O and the pyridine H is also equal to or less than the sum of the van der Waals radii, corresponding to the synthon shown in Fig. 1b. A related search of the Cambridge Crystallographic Database for co-crystals with 4-(N,N-dimethylamino)pyridine and carboxylic acids revealed only four neutral co-crystals and 54 structures corresponding to the pyridinium carboxylate as shown in Fig. 1c. A similar search for co-crystals formed between 2-aminopyridines with benzoic acids yielded 41 hits, of which 40 feature charge-assisted aminopyridinium carboxylate hydrogen-bonded co-crystals as the result of proton transfer shown in Fig. 1d. The structure that is reported to form a neutral hydrogen-bonded complex corresponds to the co-crystal formed between 2-aminopyridine and 4-aminobenzoic acid [refcode WOPCOV; Chandrasekaran & Babu, 2014 ]. Finally there is only one reported co-crystal of 2,3,5,6-tetrafluorobenzoic acid, or the corresponding 2,3,5,6-tetrafluorobenzoate, with an organic base. In that example theophylline forms a neutral hydrogen-bonded complex (Corpinot et al., 2016 ).

5. Synthesis and crystallization

2-Amino-6-bromopyridine and 2,3,5,6-tetrafluorobenzoic acid were used as supplied. An equimolar amount (0.1 mmol) of each component were added to a screw-capped vial and 3 mL of ethanol added to effect a clear colorless solution that was allowed to slowly concentrate over two weeks. A homogeneous mass of crystals was obtained.

6. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2. All hydrogen atoms were located in Fourier-difference maps. Hydrogen atoms involved in hydrogen-bonding interactions were restrained in the refinement with N—H = 0.87 (2) Å and with U_iso(H) = 1.2U_eq(N). The aromatic H atoms were included in the refinement at calculated positions with C—H = 0.95 Å and U_iso(H) = 1.2U_eq(C).

Table 2
Experimental details

Crystal data
Chemical formula	C₅H₆BrN₂⁺·C₇HF₄O₂⁻
M_r	367.11
Crystal system, space group	Monoclinic, P2₁/c
Temperature (K)	100
a, b, c (Å)	13.7230 (9), 6.5757 (4), 15.3224 (10)
β (°)	111.841 (1)
V (Å³)	1283.42 (14)
Z	4
Radiation type	Mo Kα
μ (mm⁻¹)	3.26
Crystal size (mm)	0.25 × 0.20 × 0.03

Data collection
Diffractometer	Bruker APEXII CCD
Absorption correction	Multi-scan (SADABS; Bruker, 2014 )
T_min, T_max	0.788, 1.000
No. of measured, independent and observed [I > 2σ(I)] reflections	16099, 2847, 2395
R_int	0.045
(sin θ/λ)_max (Å⁻¹)	0.641

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.026, 0.059, 1.04
No. of reflections	2847
No. of parameters	199
No. of restraints	3
H-atom treatment	H atoms treated by a mixture of independent and constrained refinement
Δρ_max, Δρ_min (e Å⁻³)	0.41, −0.32
Computer programs: SMART and SAINT (Bruker, 2014), SHELXT2018/2 (Sheldrick, 2015a ), SHELXL2018/3 (Sheldrick, 2015b ) and X-SEED (Barbour, 2001 ).

Supporting information

CCDC reference: 1893117

Crystal structure: contains datablocks I, global. DOI: https://doi.org/10.1107/S2056989019001294/pk2612sup1.cif

Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S2056989019001294/pk2612Isup2.hkl

Supporting information file. DOI: https://doi.org/10.1107/S2056989019001294/pk2612Isup3.cml

checkCIF report

Computing details top

Data collection: SMART (Bruker, 2014); cell refinement: SMART (Bruker, 2014); data reduction: SAINT (Bruker, 2014); program(s) used to solve structure: SHELXT2018/2 (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2018/3 (Sheldrick, 2015b); molecular graphics: X-SEED (Barbour, 2001); software used to prepare material for publication: X-SEED (Barbour, 2001).

2-Amino-6-bromopyridinium 2,3,5,6-tetrafluorobenzoate top

Crystal data top

C₅H₆BrN₂⁺·C₇HF₄O₂⁻	F(000) = 720
M_r = 367.11	D_x = 1.900 Mg m⁻³
Monoclinic, P2₁/c	Mo Kα radiation, λ = 0.71073 Å
a = 13.7230 (9) Å	Cell parameters from 3850 reflections
b = 6.5757 (4) Å	θ = 2.7–25.7°
c = 15.3224 (10) Å	µ = 3.26 mm⁻¹
β = 111.841 (1)°	T = 100 K
V = 1283.42 (14) Å³	Cut block, colourless
Z = 4	0.25 × 0.20 × 0.03 mm

Data collection top

Bruker APEXII CCD diffractometer	2847 independent reflections
Radiation source: fine-focus sealed tube	2395 reflections with I > 2σ(I)
Graphite monochromator	R_int = 0.045
Detector resolution: 8.3660 pixels mm^-1	θ_max = 27.1°, θ_min = 1.6°
phi and ω scans	h = −17→17
Absorption correction: multi-scan (SADABS; Bruker, 2014)	k = −8→8
T_min = 0.788, T_max = 1.000	l = −19→19
16099 measured reflections

Refinement top

Refinement on F²	3 restraints
Least-squares matrix: full	Hydrogen site location: mixed
R[F² > 2σ(F²)] = 0.026	H atoms treated by a mixture of independent and constrained refinement
wR(F²) = 0.059	w = 1/[σ²(F_o²) + (0.0271P)² + 0.3891P] where P = (F_o² + 2F_c²)/3
S = 1.04	(Δ/σ)_max = 0.002
2847 reflections	Δρ_max = 0.41 e Å⁻³
199 parameters	Δρ_min = −0.32 e Å⁻³

Special details top

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	U_iso*/U_eq
Br1	0.35106 (2)	0.41134 (3)	0.84202 (2)	0.01733 (7)
F1	0.10223 (10)	0.15160 (19)	0.49369 (9)	0.0248 (3)
F4	0.17441 (10)	0.8060 (2)	0.40322 (9)	0.0275 (3)
F2	−0.10146 (10)	0.2028 (2)	0.38905 (10)	0.0321 (3)
F3	−0.03037 (12)	0.8562 (2)	0.30158 (10)	0.0365 (4)
O1	0.27555 (11)	0.4079 (2)	0.59997 (10)	0.0188 (3)
O2	0.32636 (12)	0.4894 (3)	0.48172 (11)	0.0238 (4)
N1	0.46382 (13)	0.4100 (3)	0.72562 (12)	0.0130 (4)
H1	0.3977 (13)	0.404 (3)	0.6815 (14)	0.016*
N2	0.53368 (14)	0.4068 (3)	0.61009 (13)	0.0171 (4)
H2A	0.4701 (14)	0.424 (3)	0.5700 (14)	0.020*
H2B	0.5859 (15)	0.422 (3)	0.5919 (16)	0.020*
C4	0.57133 (17)	0.4158 (3)	0.88912 (15)	0.0170 (4)
H4	0.577544	0.416389	0.952978	0.020*
C2	0.65046 (17)	0.4148 (3)	0.77289 (15)	0.0172 (4)
H2	0.711166	0.414715	0.757094	0.021*
C12	0.25906 (16)	0.4564 (3)	0.51564 (15)	0.0155 (5)
C5	0.47591 (16)	0.4124 (3)	0.81748 (14)	0.0147 (4)
C1	0.54936 (16)	0.4114 (3)	0.70135 (15)	0.0136 (4)
C7	0.07144 (17)	0.3268 (3)	0.44649 (15)	0.0173 (5)
C3	0.66028 (17)	0.4183 (3)	0.86458 (15)	0.0182 (5)
H3	0.728259	0.422448	0.912677	0.022*
C11	0.10738 (17)	0.6527 (3)	0.40039 (15)	0.0183 (5)
C10	0.00175 (18)	0.6802 (4)	0.34810 (15)	0.0228 (5)
C8	−0.03368 (17)	0.3535 (4)	0.39179 (16)	0.0206 (5)
C6	0.14445 (16)	0.4777 (3)	0.45227 (14)	0.0157 (5)
C9	−0.06994 (18)	0.5308 (4)	0.34355 (15)	0.0241 (5)
H9	−0.142601	0.549773	0.307970	0.029*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Br1	0.01626 (12)	0.02221 (12)	0.01623 (12)	−0.00014 (9)	0.00917 (8)	0.00008 (9)
F1	0.0203 (7)	0.0188 (7)	0.0320 (8)	−0.0013 (5)	0.0061 (6)	0.0036 (6)
F4	0.0277 (8)	0.0257 (8)	0.0320 (8)	−0.0027 (6)	0.0144 (6)	0.0081 (6)
F2	0.0175 (7)	0.0384 (9)	0.0383 (9)	−0.0100 (6)	0.0081 (6)	−0.0041 (7)
F3	0.0341 (8)	0.0443 (9)	0.0320 (9)	0.0155 (7)	0.0133 (7)	0.0215 (7)
O1	0.0134 (7)	0.0282 (9)	0.0142 (8)	0.0001 (7)	0.0045 (6)	0.0028 (7)
O2	0.0150 (8)	0.0418 (10)	0.0171 (8)	−0.0004 (7)	0.0089 (7)	0.0003 (7)
N1	0.0101 (8)	0.0137 (9)	0.0154 (9)	0.0021 (7)	0.0051 (7)	0.0015 (7)
N2	0.0115 (9)	0.0234 (10)	0.0184 (10)	0.0008 (8)	0.0079 (8)	0.0027 (8)
C4	0.0196 (11)	0.0169 (11)	0.0134 (10)	0.0032 (9)	0.0049 (9)	−0.0012 (9)
C2	0.0135 (10)	0.0150 (11)	0.0231 (12)	0.0021 (9)	0.0070 (9)	0.0004 (9)
C12	0.0145 (11)	0.0167 (11)	0.0159 (11)	−0.0007 (8)	0.0062 (9)	−0.0057 (8)
C5	0.0153 (10)	0.0129 (10)	0.0173 (11)	0.0005 (9)	0.0077 (9)	0.0003 (9)
C1	0.0137 (10)	0.0093 (10)	0.0195 (11)	0.0009 (8)	0.0083 (8)	0.0018 (8)
C7	0.0174 (11)	0.0193 (11)	0.0154 (11)	0.0014 (9)	0.0062 (9)	−0.0003 (9)
C3	0.0143 (11)	0.0169 (11)	0.0191 (11)	0.0022 (9)	0.0011 (9)	−0.0012 (9)
C11	0.0192 (11)	0.0240 (12)	0.0152 (11)	−0.0010 (9)	0.0103 (9)	0.0006 (9)
C10	0.0226 (12)	0.0313 (14)	0.0159 (11)	0.0111 (11)	0.0088 (10)	0.0098 (10)
C8	0.0132 (11)	0.0312 (13)	0.0191 (12)	−0.0056 (10)	0.0080 (9)	−0.0057 (10)
C6	0.0150 (11)	0.0225 (12)	0.0118 (10)	0.0018 (9)	0.0075 (9)	−0.0036 (9)
C9	0.0129 (11)	0.0439 (16)	0.0144 (12)	0.0052 (10)	0.0037 (9)	0.0002 (10)

Geometric parameters (Å, º) top

Br1—C5	1.886 (2)	C4—C3	1.405 (3)
F1—C7	1.342 (2)	C4—H4	0.9500
F4—C11	1.355 (3)	C2—C3	1.361 (3)
F2—C8	1.349 (3)	C2—C1	1.413 (3)
F3—C10	1.345 (3)	C2—H2	0.9500
O1—C12	1.267 (2)	C12—C6	1.517 (3)
O2—C12	1.236 (2)	C7—C8	1.384 (3)
N1—C5	1.355 (3)	C7—C6	1.389 (3)
N1—C1	1.357 (3)	C3—H3	0.9500
N1—H1	0.909 (16)	C11—C10	1.383 (3)
N2—C1	1.334 (3)	C11—C6	1.383 (3)
N2—H2A	0.868 (16)	C10—C9	1.374 (3)
N2—H2B	0.866 (16)	C8—C9	1.371 (3)
C4—C5	1.360 (3)	C9—H9	0.9500

C5—N1—C1	120.04 (18)	F1—C7—C8	118.9 (2)
C5—N1—H1	118.4 (15)	F1—C7—C6	120.24 (19)
C1—N1—H1	121.5 (15)	C8—C7—C6	120.8 (2)
C1—N2—H2A	117.8 (16)	C2—C3—C4	120.9 (2)
C1—N2—H2B	120.3 (16)	C2—C3—H3	119.5
H2A—N2—H2B	119 (2)	C4—C3—H3	119.5
C5—C4—C3	117.10 (19)	F4—C11—C10	118.3 (2)
C5—C4—H4	121.4	F4—C11—C6	120.04 (19)
C3—C4—H4	121.4	C10—C11—C6	121.6 (2)
C3—C2—C1	119.5 (2)	F3—C10—C9	120.1 (2)
C3—C2—H2	120.2	F3—C10—C11	119.1 (2)
C1—C2—H2	120.2	C9—C10—C11	120.8 (2)
O2—C12—O1	126.5 (2)	F2—C8—C9	120.0 (2)
O2—C12—C6	118.28 (19)	F2—C8—C7	118.5 (2)
O1—C12—C6	115.18 (18)	C9—C8—C7	121.5 (2)
N1—C5—C4	123.21 (19)	C11—C6—C7	117.1 (2)
N1—C5—Br1	115.97 (15)	C11—C6—C12	121.12 (19)
C4—C5—Br1	120.82 (16)	C7—C6—C12	121.8 (2)
N2—C1—N1	117.94 (19)	C8—C9—C10	118.1 (2)
N2—C1—C2	122.87 (19)	C8—C9—H9	120.9
N1—C1—C2	119.18 (19)	C10—C9—H9	120.9

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1···O1	0.91 (2)	1.68 (2)	2.585 (2)	177 (2)
N2—H2A···O2	0.87 (2)	1.98 (2)	2.845 (2)	175 (2)
N2—H2B···O2ⁱ	0.87 (2)	2.02 (2)	2.854 (2)	162 (2)
C4—H4···Br1ⁱⁱ	0.95	3.13	4.017 (2)	155
C9—H9···Br1ⁱⁱⁱ	0.95	2.94	3.867 (2)	166

Symmetry codes: (i) −x+1, −y+1, −z+1; (ii) −x+1, −y+1, −z+2; (iii) −x, −y+1, −z+1.

Acknowledgements

We thank the Missouri State University Provost Incentive Fund that funded the purchase of the X-ray diffractometer.

References

Barbour, L. J. (2001). J. Supramol. Chem. 1, 189–191. CrossRef CAS Google Scholar
Bis, J. A. & Zaworotko, M. J. (2005). Cryst. Growth Des. 5, 1169–1179. Web of Science CrossRef CAS Google Scholar
Bruker (2014). SMART, SAINT and SADABS. Bruker AXS Inc., Madison, Wisconsin, USA. Google Scholar
Bruno, I. J., Cole, J. C., Edgington, P. R., Kessler, M., Macrae, C. F., McCabe, P., Pearson, J. & Taylor, R. (2002). Acta Cryst. B58, 389–397. Web of Science CrossRef CAS IUCr Journals Google Scholar
Chandrasekaran, J. & Babu, B. (2014). Private communication (Refcode WOPCOV). CCDC, Cambridge, England. Google Scholar
Corpinot, M. K., Stratford, S. A., Arhangelskis, M., Anka-Lufford, J., Halasz, I., Judaš, N., Jones, W. & Bučar, D.-K. (2016). CrystEngComm, 18, 5434–5439. CrossRef CAS Google Scholar
Desiraju, G. R. (1995). Angew. Chem. Int. Ed. Engl. 34, 2311–2327. CrossRef CAS Web of Science Google Scholar
Desiraju, G. R. (2013). J. Am. Chem. Soc. 135, 9952–9967. Web of Science CrossRef CAS PubMed Google Scholar
Groom, C. R., Bruno, I. J., Lightfoot, M. P. & Ward, S. C. (2016). Acta Cryst. B72, 171–179. Web of Science CrossRef IUCr Journals Google Scholar
Schultheiss, N. & Newman, A. (2009). Cryst. Growth Des. 9, 2950–2967. Web of Science CrossRef PubMed CAS Google Scholar
Sheldrick, G. M. (2015a). Acta Cryst. A71, 3–8. Web of Science CrossRef IUCr Journals Google Scholar
Sheldrick, G. M. (2015b). Acta Cryst. C71, 3–8. Web of Science CrossRef IUCr Journals Google Scholar
Wales, C., Thomas, L. H. & Wilson, C. C. (2012). CrystEngComm, 14, 7264–7274. Web of Science CrossRef CAS Google Scholar

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